Save! I store BM in mostly 4 oz packets. When hubby is watching LO, he splits the bag up and feeds her about half a bag, gives her a break and burps her and if she is still hungry after her break, he refills the bottle with the rest of the bag . I do keep a few 2 oz. bags as well so that if I am about to be home soon to nurse her and DD needs to eat, he can thaw a smaller amount and not have the waste.
I will use thawed BM within a few hours of thawing it. I think it is fine to put leftovers in the fridge and use for the next feeding. It's not like it's sitting out all day and I have never had any problems with that.